Re: Block level parallel vacuum WIP

On Thu, Sep 15, 2016 at 7:21 AM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> I'm implementing this patch but I need to resolve the problem
> regarding lock for extension by multiple parallel workers.
> In parallel vacuum, multiple workers could try to acquire the
> exclusive lock for extension on same relation.
> Since acquiring the exclusive lock for extension by multiple workers
> is regarded as locking from same locking group, multiple workers
> extend fsm or vm at the same time and end up with error.
> I thought that it might be involved with parallel update operation, so
> I'd like to discuss about this in advance.

Hmm, yeah. This is one of the reasons why parallel queries currently
need to be entirely read-only. I think there's a decent argument that
the relation extension lock mechanism should be entirely redesigned:
the current system is neither particularly fast nor particularly
elegant, and some of the services that the heavyweight lock manager
provides, such as deadlock detection, are not relevant for relation
extension locks. I'm not sure if we should try to fix that right away
or come up with some special-purpose hack for vacuum, such as having
backends use condition variables to take turns calling
visibilitymap_set().

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company